The invention relates to solenoid driver circuits that establish an initial actuation or pull-in current duration and a duty cycle of subsequent hold-in current pulses in a coil of a solenoid.
By way of background, there are a number of known solenoid driver circuits that operate to build up a large turn-on drive current, referred to as a "pull-in current", in the coil of a solenoid for an initial time period to thereby ensure fast, reliable actuation of the solenoid. The known solenoid drivers then supply a chopped or PWM (pulse width modulated) current, referred to as a "hold-in current", to the solenoid coil to reduce the power dissipation as the solenoid driver circuit continues to maintain the solenoid in an "on" condition.
For example, U.S. Pat. No. 4,227,230 discloses a solenoid driver in which a ramped-up drive current to the solenoid coil occurs during an initial pull-in period that is determined by an RC circuit, to initially, rapidly actuate the solenoid/valve. After the solenoid/valve is initially turned "on", then a "chopping period" occurs during which a chopped current of predetermined duty cycle maintains the solenoid in its on condition with a greatly reduced average drive current through the solenoid coil. The duty cycle of the chopped hold-in current is determined by some of the same RC circuit components that establish the duration of the ramp up or pick current.
Many prior solenoid drivers which are implemented as integrated circuits sense the current through the solenoid coil and use a closed loop feedback circuit to control the coil current by using the feedback to adjust pull-in current duration or hold-in current duty cycle. The circuitry of such solenoid drivers is undesirably complex.
It should be appreciated that solenoids and solenoid driver circuits are used in a wide range of applications and environments which impose a wide range of constraints on the speed and power dissipation of solenoid driver circuits. For example, various applications provide various power supply voltages that must be used for the solenoids and solenoid driver circuits. A typical prior art solenoid driver circuit includes complex circuitry for adjusting the pull-in current or the hold-in current, and changing one usually affects the other, so multiple circuit component changes usually are required.
Partly because of this wide range of environment and applications in which solenoid drivers are used, it is very desirable that the ramp-up duration for the pull-in current in a solenoid coil be constant and independent of the duty cycle and/or frequency of the PWM hold-in current. It also is very desirable that the pull-in current and the hold-in current produced by a solenoid driver circuit each be easily set independently of the other. These desirable features are not provided by the prior driver circuits known to the applicant. Furthermore, the number of external components required in the closest prior art solenoid driver circuits is larger than desirable.
Those skilled in the art will appreciate that in an integrated circuit the resistances of resistors and the capacitances of capacitors often vary by about .+-.20% from nominal values, and that it would not be acceptable for the hold-in current duty cycle of a solenoid driver circuit to be subjected to a unit-to-unit variation of anywhere near .+-.20%. This probably is why the circuitry that determines the hold-in current duty cycle in prior solenoid drivers has not been designed to be settable merely by selecting the value of a single component.